1. Field of the Invention
The present invention relates to catalyst particles used in the purification of automobile exhaust gas, in fuel cells and in environment conservation and, more particularly, to catalyst particles having sizes of a nanometer order.
2. Description of the Related Art
Noble metals such as Pt, Pd and Rh are used, for example, as catalysts for removing toxic components such as HC, CO and NOx from the exhaust gas of an automobile. These catalytic noble metals are supported in the form of particles on the surface of a support such as alumina, in order to increase the area of contact with the exhaust gas to thereby remove the toxic components.
Recently, as the regulations on the automobile exhaust gas have become increasingly demanding, the exhaust gas purifying catalysts are required to remove the toxic components more efficiently. On the other hand, as the catalysts for fuel cells (for example, the catalysts for the reaction of hydrogen and oxygen and the catalysts for reforming methanol) and the catalysts for environment purification are required to have improved purification performance and functions, there have been increasing demands for catalysts having higher catalytic activity.
Measures to improve the efficiency of a noble metal catalyst include increasing the contact area with toxic components by forming the noble metal catalyst as fine particles. However, conventional methods of supporting the catalyst only allow noble metal particles having sizes of submicron order (several hundreds of nanometers), impeding further improvements in the specific surface area of the catalyst. Thus there is increasing demand for fine particles of noble metal catalyst having sizes of nanometer order (the term xe2x80x9cnanometer orderxe2x80x9d refers to sizes below about 100 nm).
With the background described above, significant efforts have been made for the development of noble metal particles of nanometer order size, that have a larger contact area, in order to improve the activity further. For example, Published Japanese Translations of PCT International Publication No. 2000-510042 discloses a constitution of noble metal particles of nanometer order size supported on xcex3Al2O3 by a sputtering process. Japanese Unexamined Patent Publication (KOKAI) No. 2000-15098 discloses a constitution where fine particles having sizes of 20 nm or less are supported on a support made of Al2O3 or the like by concurrent electron beam irradiation.
According to the Publications mentioned above, although the fine noble metal particles of nano-order size can be deposited on the support, no specific description is given as to the surface condition of the noble metal fine particles (particle shape, specificity, etc.).
Moreover, the gas purifying catalysts generally have activities to selectively decompose particular types of toxic substances, rather than universal activity to decompose many toxic substances. Accordingly, when exhaust gas including many types of toxic substances must be neutralized effectively, various types of catalyst may be selected for the toxic substances of interest and used in combination. However, there is no description of the combination of catalysts in the Publications mentioned above.
In consideration of the problems described above, the present invention has an object of providing catalyst particles having higher catalytic activities that are effective for a plurality of types of substance, and a method of manufacturing the catalyst particles.
In order to achieve the object described above, a first aspect of the invention provides catalyst particles that comprise base particles (1) that consist of one kind of single material fine particles or two or more kinds of solid solution fine particles having a primary particle diameter of a nanometer order, and one or more kind of a metal or a derivative thereof that covers at least a part of the base particles.
The primary particle size refers to the diameter of a single particle. One kind of single material fine particles refers to fine particles made of one kind of element or compound. Two or more kinds of solid solution fine particles refer to fine particles made of two or more kinds of element or compound forming a solid solution. The term solid solution used in the present invention includes the state of substances A and B being mixed, and the state of substances A and B forming a new structure through reaction with each other.
The catalyst particles of the present invention comprise the base particles having sizes of a nanometer order and one or more kind of metal, or a derivative thereof, that covers at least a part of the surface of the base particles.
Thus it is made possible to form the single catalyst particle having a size of a nanometer order (about 100 nm or smaller), resulting in larger specific surface area and higher activity than those of the simple noble metal particles of nano-order size of the prior art.
According to the invention, as the base particles can provide catalytic activity while the base particles and the one or more kind of metal or derivative thereof can be selected so that both components show catalytic activities for different substances, catalytic activities for a plurality of substances can be achieved with one type of catalyst particle. Therefore, the invention can provide catalyst particles that have higher activity and are capable of showing activity for a plurality of substances.
The base particles (1) can be selected from among metal oxides, metal carbides and carbon materials. Specifically, the metal oxide may be either one kind of single material fine particles selected from among oxides of Ce, Zr, Al, Ti, Si, Mg, W and Sr and derivatives thereof, or two or more kinds of solid solution. The metal carbide may be SiC or a derivative thereof. The carbon material may be graphite. The metal oxide may be expressed as MxOy, where M represents the metal while x and y represent variable values.
The one or more kind of metal, or a derivative thereof, that covers at least a part of the base particles (1) may be used in the form of either particles or layer. In the case of particles, they are preferably ultra-fine particles of 50 nm or less in diameter. Particle size of 50 nm or larger makes it difficult to cover the surface of the base particles of a nanometer size.
In the case of layer, on the other hand, it is preferable to form a coating film (2) comprising one to thirty single atom layers. When the surface coating layer consists of more than 30 single atom layers, it is difficult to keep the size of a single catalyst particle within nanometer order, or the surface coating layer itself becomes like a particle, thus resulting in a smaller specific surface area.
The one or more kind of metal, or a derivative thereof, that covers at least a part of the base particles (1) has purity of preferably 99% or higher.
Furthermore, it is preferable that a part, not the entire surface, of the base particles (1) is covered by the one or more kind of metal or a derivative thereof. This constitution makes it possible to effectively utilize the characteristics of the base particles on the surface of the base particles exposed without being coated with the one or more kind of metal, or a derivative thereof, in a case where the base particles have catalytic activity.
For the one or more kinds of metal, or a derivative thereof, one kind of single material fine particles selected from among noble metals such as Pt, Rh, Pd, Au, Ag and Ru and oxides thereof, or two or more kinds of solid solution, can be used.
In case the catalyst particles are used at a high temperature of around 1000xc2x0 C. in an application such as purification of exhaust gas, sintering of the particles may be caused by the heat thus resulting in a decreased surface area that has reactivity, due to bonding of particles with each other and/or mobilization of the surface coating layer that causes coagulation into coarse particles.
A second aspect of the invention aims at solving the problem of sintering, and provides a constitution of the catalyst particles described in the first aspect of the invention where the base particles (1) have, on the surface thereof, an anti-sintering agent (3) made of a metal or a metal oxide having melting point of 1500xc2x0 C. or higher.
The anti-sintering agent may be present either on the surface of the one or more kind of metal or a derivative thereof that covers the base particles, or on the surface of the base particles not covered by the one or more kind of metal or a derivative thereof.
According to the invention, as the base particles have, on the surface thereof, the anti-sintering agent that remains stable even when used at a high temperature near 1500xc2x0 C., the catalyst particles that have excellent durability against high temperature can be made, while preventing the bonding of particles with each other and the bonding of the one or more kind of metal or derivative thereof, thereby mitigating the decrease in the specific surface area that has reactivity.
In order to make the catalyst particles each having a size of a nanometer order, it is preferable to form the anti-sintering agent (3) in a layer having a thickness of one to thirty atom layers, or as ultra-fine particles having diameters smaller than 50 nm.
The anti-sintering agent (3) may be either one kind of single material selected from among oxides of Al, Mg, Ca, Ce, Sr, Zn, W and Mo and derivatives thereof, or a solid solution composed of two or more kinds of the single material.
It is preferable to include the anti-sintering agent (3) in a proportion of 1 to 20% by weight of the one or more kind of metal or a derivative thereof that covers at least a part of the base particles (1). When the proportion is less than 1% by weight, sufficient effect of the anti-sintering agent cannot be achieved. When the proportion is more than 20% by weight, the specific surface area having catalytic activity is greatly decreased by the presence of the anti-sintering agent.
A third aspect of the invention is a method of manufacturing the catalyst particles described above, where two or more kinds of raw materials are evaporated at the same time in a vacuum chamber (10), so as to form the base particles (1) that are covered, on at least a part of the surface thereof, with the one or more kind of metal or a derivative thereof.
The catalyst particles described above can be satisfactorily manufactured by employing a simultaneous evaporation process such as the manufacturing method of the invention.
It is preferable to place an evaporation source (18b) of the raw material that makes the one or more kind of metal or a derivative thereof at a position higher than an evaporation source (18a) of the raw material that makes the base particles (1) in the vacuum chamber (10). This enables the one or more kind of metal or derivative thereof to grow hetero-epitaxially on the base particles that are formed earlier upon being evaporated at a lower position.
It is preferable to evaporate the raw materials by resistive heating method in order to improve mass-producibility and crystallinity of the material.
The numerals given in parentheses for the means described above represent an example of correspondence to specific means in the preferred embodiments described later.